Photovoltaic modules, such as solar panels, convert solar energy to electricity. The cells within a module are typically connected in series by means of copper ribbons. The copper ribbons are terminated in a specially designed junction box where the current is collected and transferred to a pair of short cables that end in connectors. An array of photovoltaic modules is typically interconnected using the cables terminated in the junction boxes.
The cells of photovoltaic modules are connected in series so that the sum voltages of the cells is at a useable voltage level. When exposed to light, photovoltaic cells generate electricity. When shaded, however, photovoltaic cells not only cease to produce electricity, but also become poor conductors of electricity. Because the cells are connected in series, a single shaded cell creates a bottleneck for all the other cells. When that happens, the shaded cell turns into a resistance heater, which destroys the power of its neighbors. The shaded cell can also heat up to such an extent that it can destroy the module, expose dangerous voltage carrying conductors and in some cases cause a fire. Thus a single shaded photovoltaic cell may render a PV installation useless.
To avoid this, bridging diodes are used. The bridging diode is wired in parallel to the string of cells in such a way that when the cells all behave normally, the diode is in reverse bias condition, no current flows over the diode. If one or more cells become shaded, the shaded cells produce a voltage drop instead of creating a voltage increase, so that the bias voltage over the diode changes polarity and the diode is now in forward mode, conducting the current. In doing this, the diode effectively bridges the shaded cells and prevents catastrophic failure as well as a means to conduct the current of the un-shaded cells and therefore still generating power. Those bypass diodes are typically built into the junction box. The forward voltage drop over the diodes, however, generates heat within the diodes which must be dissipated to prevent the diode from overheating. If the heat generated is not properly dissipated, the diode will reach a thermal runaway status and be destroyed in the process. In this way, the failure of a single diode may render an entire string of panels useless. The heat generated by the diodes within a junction box may not be conducted to the module, since this may cause a hot spot and contribute to a thermal runaway situation of the photovoltaic cells in close proximation. The junction boxes also must meet certain industry standard requirements regarding heat and temperature.
Therefore, a need exists for a photovoltaic junction box that can accommodate higher currents and more effectively dissipate heat while also meeting the temperature requirements of industry standards.